Ice accumulation on aircraft surfaces has been a problem since the inception of the aviation industry. The accumulation of ice has four main effects which are all negative and in some instances catastrophic. First, aerodynamic performance is severely restricted resulting in a loss of lift and increase drag. Second, the accumulation of ice increases the aircraft weight. Third, the accumulation of ice will impair or restrict the movement of control surfaces. Fourth, the ice may be ingested into the engine or other system intakes terminating engine operation.
Recently, interest in aircraft icing has been greatly heightened with an increase in industry and public awareness of the hazards associated with this problem. Although the detrimental effects of ice build up on aircraft performance has been generally well acknowledged, difficulties in predicting or measuring ice accumulation on aircraft has prevented rigorous and reliable procedures for flight crews both on the ground and in the air to minimize this problem.
The problem of aircraft icing occurs in two broad categories. First, inflight icing occurs on the leading edge of the airfoil. This type of ice build up is common and is handled by pilot observations or pilot awareness or suspicions of impending icing conditions. In many aircraft, the leading edges of the wing are heated by engine bleed air at temperatures of up to 250.degree. C. Engine air bleed is normally done at regular intervals when icing conditions are likely regardless as to whether any ice has accumulated. A percentage of engine air is required to be used to heat the aircraft wing rather than for propulsion purposes. It is very inefficient to bleed engine air when no ice has accumulated on the aircraft surface.
The second category of aircraft icing is ground icing. Ground icing occurs over the top of the aircraft surface when the aircraft is standing. Icing on the leading 10% of the wing has the most critical aerodynamic effect. This type of ice accumulation is handled by the application of de-icing or anti-icing fluids. The problem is amplified since de-icing depends not only on how well the de-icing was undertaken but also whether ice has re-accumulated since de-icing.
On current commercial aircraft, pilots have no reliable way of judging the amount of ice accumulated on the surface of the aircraft both inflight and on the ground. Further, pilots have no means of assessing the status of the de-icing or anti-icing fluids which may have been applied in accordance with current flight procedures. Pilots are accordingly faced with difficult decisions on a regular basis in order to maintain flight schedules.
Several devices have been proposed which are designed to detect the presence of ice which has accumulated on the aircraft surface. One such device will vibrate an aircraft surface at a known frequency. When the aircraft surface vibrates at a different frequency, the presence of ice has been detected.
Still other devices have been proposed which detect the presence and thickness of ice on an aircraft surface. Such devices have been described in detail in U.S. Pat. No. 4,766,369, Weinstein. This device uses two capacitive gauges and a temperature gauge. The ratio of the voltages of sense side of the capacitive gauges determines the thickness of the ice present.
Although, these devices may detect the presence of ice on an aircraft surface it cannot detect the presence of substances other than ice such as snow, slush, de-icing fluid or din. In fact, there are no known devices which can detect the presence of snow on an aircraft surface.
Devices and analytical techniques exist for non-intrusive interrogation of materials to deduce their physical properties. Dielectric sensors and analytical techniques measure the spatial profile of permittivity of a material by multiple wavelength interrogation. A spatially periodic field is imposed upon the material via a first electrode under the control of a wavelength controller. A second or sense electrode is then used to measure the effect of the material on the charge induced by the first electrode in response to the periodic field. By varying the wavelength, spatial distribution of complex permittivity is deduced as a function of the temporal frequency. The physical properties of the material can then be deduced.
Such devices are used to monitor material changes such as the outgassing of solvents from paints, the removal of moisture from coatings, the diffusion of dopants into semi-conductors and the deposition of materials. Such devices and techniques are more fully described in U.S. Pat. No. 4,814,690, Melcher, et al. and U.S. Pat. No. 5,015,951, Melcher.
Initially, it was believed that such devices and techniques would be useful in the detection of ice accumulation on an aircraft surface. However when attempts were made using such sensors to detect accumulation of ice on aircraft surfaces, the analytical techniques of Melcher, et al. were found to be highly unstable and could not in real time accurately and reliably detect, identify and measure the thickness of the various contaminants accumulating on the aircraft surface.
One of the problems of the approach of Melcher et al. is that the electric potential along a planar electrode array must be sufficiently defined and known at all times. The electric potential is required so that the theoretical models can be used to predict the spatial permittivity profile of the measured substance. However, the electric potential varies depending on the electrical properties of the substance being measured. Since the various substances which can accumulate on the aircraft surface are not known beforehand, Melcher et al., was unsuitable for use as a substance detector.
Melcher also requires sampling to be of a laboratory-quality so that the non-analyticity problems such as irregularity of the surface and complex structures could be avoided. Even with approximations, real time data processing was not possible. Data analysis using Melcher's techniques is normally in the order of hours.
Dielectric sensors measure the effects that the interrogated substance has on the capacitance of the imposed field. The problems of air gaps on dielectric sensors are well known (see U.S. Pat. No. 5,045,798, Hendrick and U.S. Pat. No. 5,095,278, Hendrick). Air gaps severely limit the sensors' ability to measure the dielectric properties since air and a vacuum have the lowest theoretically possible permittivity. Further, air also induces noise into the capacitance measurement.